A polymer having a conjugated double bond (i.e. π-conjugated polymer) does not show a conductivity by itself; however, if an appropriate anionic molecule is doped therein, it can express a conductivity, thereby giving a conductive polymer material (i.e. conductive polymer composition). As to the π-conjugated polymer, polyacetylene, (hetero) aromatic polymers such as polythiophene, polyselenophene, polytellurophene, polypyrrole, and polyaniline; a mixture thereof, etc., are used; and as to the anionic molecule (dopant), an anion of sulfonic acid type is most commonly used. This is because a sulfonic acid, which is a strong acid, can efficiently interact with the aforementioned π-conjugated polymers.
As to the anionic dopant of sulfonic acid type, sulfonic acid polymers such as polyvinyl sulfonic acid and polystyrene sulfonic acid (PSS) are widely used (Patent Document 1). The sulfonic acid polymer includes a vinylperfluoroalkyl ether sulfonic acid typified by Nafion (registered trademark), which is used for a fuel cell.
Polystyrene sulfonic acid (PSS), which is a sulfonic acid homopolymer, has a sulfonic acid as a repeated monomer unit in the polymer main chain, so that it has a high doping effect to the π-conjugated polymer, and also can enhance water dispersibility of the π-conjugated polymer after being doped. This is because the hydrophilicity is kept due to the sulfo groups excessively present in PSS, and the dispersibility into water is therefore enhanced dramatically.
Polythiophene having PSS as a dopant exhibits high conductivity and can be handled as an aqueous dispersion, so that it is expected to be used as a coating-type conductive film material in place of ITO (indium-tin oxide). As mentioned above, however, PSS is a water-soluble resin, and is hardly soluble in an organic solvent. Accordingly, the polythiophene having PSS as a dopant also has a high hydrophilicity, but a low affinity to an organic solvent and an organic substrate, and thus, it is difficult to disperse it into an organic solvent and to form a film onto an organic substrate.
Besides, when the polythiophene having PSS as a dopant is used in, for example, a conductive film for an organic EL lighting, a large quantity of water tends to remain in the conductive film and the conductive film thus formed tends to absorb moisture from an outside atmosphere since the polythiophene having PSS as a dopant has an extremely high hydrophilicity as mentioned above. As a result, the problems arise that the luminous body of the organic EL chemically changes, thereby the light emitting capability is deteriorated, and that water agglomerates over time and defects are caused, which results in shortening of the lifetime of the whole organic EL device. Furthermore, there arise other problems in the polythiophene having PSS as a dopant that particles in the aqueous dispersion becomes large, the film surface becomes rough after the film formation, and a non-light emitting region, called dark spot, is caused when used for the organic EL lighting.
In addition, since the polythiophene having PSS as a dopant has an absorption at a wavelength of about 500 nm in the blue region, in the case that this material is used as a film coating a transparent substrate such as a transparent electrode, there arises another problem that when the conductivity required for the device to function is made up by the solid concentration or the thickness of the film, transmittance of the film is affected.
Patent Document 2 discloses a conductive polymer composition composed of a conductive polymer which contains a π-conjugated polymer formed of a repeating unit selected from thiophene, selenophene, tellurophene, pyrrole, aniline, and a polycyclic aromatic compound, and a fluorinated acid polymer which can be wetted by an organic solvent and 50% or more of which is neutralized by a cation; and it is shown that an aqueous dispersion of the conductive polymer can be obtained by combining water, a precursor monomer of the π-conjugated polymer, the fluorinated acid polymer, and an oxidant, in any order.
However, in such a conventional conductive polymer, particles are agglomerated in the dispersion immediately after synthesis. Also, if an organic solvent served as a conductive enhancer is added thereto to give a coating material, the agglomeration is further facilitated, so that the filterability thereof is deteriorated. If the coating material is applied by spin coating without filtration, a flat film cannot be obtained due to the effect of the particle agglomeration; and as a result, the problem of coating defect is caused.
Moreover, the polythiophene having PSS as a dopant can also be used as a hole injection layer. In this case, the hole injection layer is provided between a transparent electrode such as ITO and a light-emitting layer. The hole injection layer does not require high conductivity since the under transparent electrode ensures the conductivity. For the hole injection layer, no occurrence of dark spot and high hole-transporting ability are required.